June 29, 2011

Diesel Particulate Filter (DPF Filter)

Diesel Particulate Filters were added to vehicle emissions legislation in 2009.

A Diesel Particulate Filter (sometimes referred to as a DPF Filte) removes the diesel particulate matter (or soot) from the exhaust gas of a diesel vehicle, therefore reducing particulate emissions. Diesel Particulate Filters usually remove more than 85% of the soot.

How Does a Diesel Particulate Work?

Diesel particulate filters work by forcing engines gasses to flow through a complex ceramic honeycomb structure. The channels of the filter are blocked at alternate ends, so the gasses are forced to flow through the cell walls in order to exit the filter. Also the cell walls are porous, so the particulate matter gets deposited on the walls, while the gasses are allowed to pass through. This ensures that only the clean exhaust gasses can exit, and the particulate matter is trapped in the filter.

Passive Regeneration

Passive regeneration takes place automatically on motorway-type runs when the temperature of the exhaust is high, but problems can occur when cars don’t reach this sort of temperature on a regular basis (which many cars don’t). Therefore manufacturers have come up with a solution called “Active Regeneration”.

Active Regeneration

Active Regeneration takes place when the soot level in the filter builds up to around 45%. The ECU makes small adjustments to the fuel injection timing and increases the exhaust gas temperature. This increases the exhaust temperature which then initiates the regeneration process, burning away the soot trapped in the DPF.

Types of Filter

Cordierite filters provide excellent filtration efficiency and are relatively inexpensive. As the name suggests, they are made from a ceramic material called cordierite, which is also used in catalytic converters. The only problem with cordierite wall flow filters is the low melting point, which has been known to melt down during filter regeneration.

Silicon Carbide Wall Flow Filters are the second most common filter. They have a far higher melting point than cordierite filters although they are not as thermally stable.

Ceramic Fibre Filters are manufactured by mixing various types of ceramic fibres, to create a porous media. This can then be formed into any shape to suit various applications. The main advantage of ceramic fibre filters is that they have a lower back pressure.

Metal Fibre Flow Through Filters are made by weaving metal fibres into a monolith. They have the ability to pass electrical current through, which can heat the monolith for regeneration purposes. This type of filter tends to be more expensive.

Diesel Particulate Filters need to be emptied regularly to maintain performance and prevent further problems. If you ignore the warning lights and keep driving in a relatively slow, stop/start pattern soot loading will continue to build up until around 75% when you can expect to see other dashboard warning lights illuminate too. At this point driving at speed alone will not be sufficient and the car will have to go to a dealer for regeneration.

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